Which timer(s) do you want to modify? You realize that different timers affect different pairs of PWM pins, right? Altering the behavior of Timer0 has side effects, eg, millis(), that you may not want. The other timers are fair game as long as they aren't used by a library you've included in your sketch.

here's some actually useful information. I didnt read all the posts, but yes you can change the pwm frequencies. There are 8bit and 16bit pwm options. I really haven't looked into it too much since i just had to change it for one job.

http://arduino.cc/en/Tutorial/SecretsOfArduinoPWMis a great article. Really you want phase correct pwm if possible, unless you need pwm above about 30khz.

for the 8 bit timers, start reading page 94 of the 328P datasheetyou can set N on the equations by looking at page 110. Really the whole 8bit pwm section is only 9 pages.

If you need more resolution look at the 16bit.

The closest you can get to 500hz without using the cpu's time is 490hz.